1. Field of the Invention
The invention relates to gas diffusion structures such as gas diffusion electrodes and gas diffusion electrode backings for electrochemical applications, and to methods for producing the same.
2. Description of Related Art
Gas diffusion structures are increasingly used in electrochemical applications such as fuel cells and electrolyzers, particularly in those applications making use of ion-exchange membranes as separators and/or as electrolytes. A gas diffusion structure (also called a “gas diffuser”) is normally comprised of a web, acting as a support, and of coating layers applied on one or both sides thereof. The coating layers have several functions, the most important of which are providing channels for water and gas transport and conducting electric current. Coating layers, especially the outermost ones, may also have additional functions such as catalyzing an electrochemical reaction and/or providing ionic conduction, particularly when they are used in direct contact with an ion-exchange membrane. For most applications, it is desirable to have a porous current conducting web (such as a carbon cloth, a carbon paper or a metal mesh) coated with current conducting layers. It is also desirable that the channels for water and for gas transport be separate channels, characterized by different hydrophobicity and porosity.
It is known in the art that gas diffusers may be advantageously provided with two different layers, an inner and an outer coating layer, having different characteristics: for instance, U.S. Pat. No. 6,017,650 discloses the use of highly hydrophobic gas diffusers coated with more hydrophilic catalytic layers for use in membrane fuel cells. U.S. Pat. No. 6,103,077 discloses methods for automatically manufacturing such type of gas diffusion electrodes and electrode backings with industrial coating machines. In the cited documents, the coating layers are composed of mixtures of carbon particles and a hydrophobic binder such as PTFE, and the methods of obtaining a diffusive and a catalytic layer with distinct characteristics comprise the use of different relative amounts of carbon and binder materials and/or the use of two different types of carbon in the two layers.
Also, gas diffusers having two layers with different porosity are known in the art: DE 198 40 517, for instance, discloses a bilayer structure consisting of two sub-structures with different porosity. Surprisingly, the layer with higher porosity and gas permeability is the one in contact with the membrane, while the less porous and permeable layer is the one that contacts the web. There is, in fact, a general understanding that a desirable porosity gradient should provide a less permeable structure for the layer in contact with the membrane, for example as disclosed for the catalytic layer of WO 00/38261. Although in such case, the porosity gradient is not obtained in a gas diffuser structure but only in a very thin catalytic hydrophilic layer in direct contact with an ion-exchange membrane, the general teaching that a less porous geometry is desirable for the side of a gas-fed electrode structure which has to be coupled to a membrane electrolyte may be regarded as common knowledge in the art.
Such type of bilayer gas diffusion structures show adequate performances in most applications; however, there are a few critical applications in which the gas diffuser architecture of the prior art does not meet the gas and water transport requirements to a sufficient extent. Particularly critical applications comprise, for instance, membrane fuel cells operating at relatively high temperature (close to or higher than 100° C.) and oxygen-depolarized aqueous hydrochloric acid electrolyzers, especially if operating at high current density or if depolarized with air or other depleted oxygen-containing mixtures instead of pure oxygen. In these cases, the optimum gas transport and water management are not achieved by means of a simple bilayer gas diffusion structure.